Sound reproduction system



Oct. 26, 1937. E. s. PURINGTON 5 SOUND REPRODUCTION SYSTEM 7 Filed July22, 1935 2 Sheets-Sheet l 11v VENTOR Ellison S. Puringfon f4 TTORNEY vPatented Oct. 26, 1937 UNITED STATES SOUND REPRODUCTION SYSTEM EllisonS. Purington, Gloucester, Mass, assignor to John Hays Hammond, Jr.,Gloucester, Mass.

Application July 22,

16 Claims.

This invention relates to sound amplifying systems and more particularlyto a system of the above type having volume expansion and scratchelimination characteristics.

An object of the invention is to provide a novel and improved system ofthe type above indicated.

A feature of the invention consists in the provision of a scratch filtercomprising a capacity resistance network having means to vary theeffectiveness of the capacity so as to control the frequencydiscrimination characteristics of the filter. In the form shown this isaccomplished by means of a space discharge device having its spacecurrent path connected in series with the capacity elements of thefilter so that the effectiveness of the capacity varies in accordancewith the impedance of the device. For scratch elimination purposes, itis usually desirable to provide a filter which discriminates against thehigher frequencies. The capacities and space discharge devices are,accordingly, disposed in series in the shunt leg of the filter so that,when the space discharge devices are of low impedance, the filterdiscriminates more against the high frequencies than when the devicesare of high impedance. The invention provides for controlling theimpedance of the space discharge devices in accordance with the strengthof the input signal or in accordance with the high frequency componentof the input signal. In this way, the filter is caused to preventpassage of scratch frequencies except when they are accompanied bysignals sufficient in intensity to mask the same.

Another feature of the invention consists in the provision of a filterof the above type containing only capacity and resistance elements sothat, when the capacity elements are ineffective, the filter does notappreciably affect the frequency transmission characteristics of thesystem.

Another feature consists in the arrangement of the shunt capacities andin push-pull relationship uneven operation is avoided.

Another feature consists in the provision of means to supply acomparatively high fixed bias potential to the space discharge deviceswhen desired so as to cause them to have a sufficiently high impedanceto render the capacity elements substantially ineffective for filterpurposes.

Another feature consists in the combination of the scratch filter withan amplifier having volume expansion characteristics and the provisionof means to reduce the effectiveness of the scratch filter and to reducethe expansion characteristics of the amplifier when low signal volumesare space discharge devices so that distortion and 1935, Serial No.32,684

and shunt resistance are adapted to be connected in circuit with thispotentiometer so as to'reduce the control potential which is derivedtherefrom.

Other features consist in the various new and original details ofconstruction and. combinations of parts hereinafter more fully setforth.

Although the novel features which are believed to be characteristic ofthis invention will be particularly pointed out in the claims appendedhereto, the invention itself, as to its objects and advantages, the modeof its operation and the manner of its organization may be betterunderstood by referring to the following description taken in connectionwith the accompanying drawings forming a part thereof, in which Figure 1illustrates diagrammatically the system as applied to the reproductionof sound from the sound record of a talking moving picture projector andphonograph;

Figure 2 depicts a curve showing the effect of various frequencies onthe control of the filter circuit; and

Figure 3 depicts a curve showing the relative transmissioncharacteristics of the filter circuit for various values of spacecurrents in the controlling elements.

In the following description and in the claims, parts will be identifiedby specific names for convenience, but they are intended to be asgeneric in their application to similar parts as the art will permit. 7

Referring to the accompanying drawings, and more particularly to Figure1, a phonograph pickup It is shown which is connected to two con-' tactsof a double pole double throw switch I I, the other contacts of whichare connected to the circuit of a photoelectric cell I3 includingbattery l5. Light from a lamp I6 is focused by means of a lens I!through an aperture l8 in an aperture plate l9 upon the sound record ofa moving photographic film 20. The light which passes cuit of thisamplifier is connected through a potentiometer 23 to the input circuitof a second space discharge amplifier 25. The output circuit of theamplifier 25 includes a choke coil 26, a blocking condenser 21, a'filter network 28 and two potentiometers 29 and 30. Connected across thepotentiometer 30 is a tone compensation circuit including a resistor 3|,a choke 32, and a condensers 49 and 4|, resistors 42 and 43, the.

condenser 33.

The contact of the potentiometer 29 is connected to the primary ofa'transformer 3 5; the V f secondaryof which is connected throughresistors 36 to the input circuit of a push-pull amplifier 31 comprisingtubes 31a. Between the secondary of the transformer 35 and the. inputcircuit of the amplifier 31. is a balanced type filtering networksubstantially of the resistancecondenser type which includes resistors38 and The bridging arms of this filter include tube impedances ofvacuum tubes 44 and 46, and choke coils 41 and 48. If found desirableresistors may be substituted for the choke coils 4'! and 48 or chokecoils may be substituted for the resistors 36. A condenser 49 isconnected between the mid-point of the resistor 39 and the cathodes ofthe push-pull amplifier '31.

The output circuit of the push pull amplifier 31 is connected through atransformer 50 to the input'circuit of a power push-pull amplifier 5I.

A resistor 52 is connected across the primary of the transformer 50 tomaintain the frequency characteristics substantially independent of thegain ratio characteristics.

The output circuit of the power amplifier 5!. is connected through atransformer 53 to the moving coil of a loud speaker 55, the field coil58 of which is connected across a battery 51 which is also used as asource of plate supply for the various space discharge devices.

The output circuit of the amplifier 25 is also 7 connected through asecond stoppage condenser 60 to the input circuit of a space dischargeamplifier-limiter tube SI, of the duplex-diode triode type, and througha resistor 62 to the mid point oftwo potentiometers 63 and 64 and toground. The potentiometers 63 and 64 are operated in unison with thepotentiometers 29 and 39. a

The output circuit of the amplifier 6| includes condensers B6, 61, and68, resistors in, II, I2, 13, and I4, and potentiometers I8 and IT, and,two single pole double throw switches I8 and I9 for either shortcircuiting the resistors l8 and I3, respectively, or for connecting theresistor H in parallel with the resistor I8 and the potentiometer I6 andfor connecting the resistor I4 in parallel with the resistor I3 and thepotentiometer 'I'I. The movable contact of the potentiometer 'IB 'isconnected in the input circuit of a space discharge amplifier 85, theoutput circuit of which is connected to the primary of a transformeriii. The secondary of the transformer 8| is connected to a rectifiercircuit including av rectifier tube 82, a condenser 83, and a resistor84. The positive side of the resistor 84 is connected through a resistor86 to the center tap of the secondary of the transformer 35, to thecenter tap of the resistance 39 and to the diodes 6Ia.

transformer' 9i to the input circuit of a space discharge amplifier 92.The output circuit of this amplifier is connected through a secondautotransformer 93 to a rectifier circuit which includes a condenser 95,a rectifier tube 96, and a resistor 9i. The'positive side of theresistor QIis connected to the movable contact of the potentiometer 53.The negative side of this resister 97 is connected through a resistor 98to the grids of the tubes 44 and 45. Between the cathodes and grids ofthese tubes and 46 is connected a condenser 99.

A bleeder system including resistors 58, I89,

7 H9, III and potentiometers 63 and G4 is bridged across the platesuppiy battery 51 to supply various operating potentials for theelectrodes of the tubes. The bleeder net-work is suitably by passed bycondensers Illa to minimize signal currents in the bleeder resistors. 7The cathodes of the tubes 44 and 45 are connected together and to thepositive side of a meter H2. The cathodes of the tubes 31a are connectedtogether and to the positive side of a meter H3. The negative sides ofthe meters ll2iand 'II3 are connected together and to the junction ofthe resistors I09, IIS and HI. The space currents of the tubes 44, 5 and31a pass through the meters H2 and II3, and', combined with the bleedercurrents, pass to ground through the resistorslio and III andpotentiometers 63 and 64; Y

A switch IOI is provided for establishing, when desired, a suitablefixed bias on the tubes 31a. byconnccting the mid-point of the resistor39 to the negative side of the resistor II I, which is also connected tothecathode of the amplifierlimiter tube 6!. A switch I82 'is providedfor grounding the grids of the tubes 44 and 48, when desired, therebyestablishing a relatively high bias on these tubes. These switches I9!and I62 render the rectifiers inoperative and cause normal functioningof the system by causing the push-pull amplifier 31 to operate as anormal amplifier and by rendering the tubes 44 and .46 non-conductive.

In the operation of the embodiment of the invention shown in theaccompanying figure; the energy received by the transformer 2I passes tothe amplifier 22 where it is amplified and fed through thepotentiometer23 to the second amplifier 25, where it is further amplified and themajor portion is fed through the blocking condenser 21 and filtercircuit 28 to the volume control potentiometers 29 and 39. The filtercircuit 28 provides means for suppressing the very between the variousfrequencies is varied in such a way as to maintain the apparent qualitysubstantially constant and independent of the volume level. In this waythe circuit is caused to diate frequencies so that the energydistribution compensate for the different response characteristics ofthe human ear to different frequencies. The ear, for example, is moresensitive to intermediate frequencies in the audible range than to thehigh and low frequencies. This discrimination is greatest at low volumesand is less evident as the higher volumes are reached. The tone controlcircuit is accordingly designed to compensate for this characteristic sothat the volume can be altered without changing the apparent quality ofthe selection.

Energy is fed from the potentiometer 29 to the input circuit of thepush-pull amplifier 31, the amount available for this purpose beingdetermined by the setting of the potentiometer 29. This energy isamplified by the amplifier 31 and is fed through the transformer 55 tothe pushpull power amplifier 5| where it is further amplified. Theoutput of the power amplifier 55 is then .fed through the transformer 53to the loud speaker 55 where it is reproduced as speech or music in awell known manner. For controlling the operation of the amplifiercircuits above mentioned, some of the energy from the amplifier 25passes through the stoppage condenser 60 to the tube M. The condenser 66and resistor 62 are so chosen that only a small amount of the totaloutput energy from the amplifier 25 is diverted for control purposes.The constants of these elements are further chosen so that suitablediscrimination against very low tones will be made as determined by theacoustical efiect produced and are selected so as to prevent the lowertones, by reason of their greater energy content, from exercising unduecontrol. Energy Produced by amplification by the tube 6! is transmittedto condensers 66, 6'7, and d8, resistors iii, H, l2, l3, and M andpotentiometers i6 and Ti. These condensers and resistors are so chosenas to discriminate against low and intermediate frequency tones with theexception of condenser 66 which will pass these tones to a considerabledegree.

Energy for controlling the potential across condenser $9 for dynamicmultiplier purposes is derived from potentiometer 16, while energy forproducing voltage across condenser 99 for automatic scratch filterpurposes is obtained from potentiometer ll. Switches 18 and iii areprovided so that the energy available for the potentiometers l3 and Timay be made large or small without substantial change of thetransmission characteristics of the filtering network.

Energy for dynamic multiplication control purposes is derived frompotentiometer l6 and is amplified by tube 85. The amplified energy fromthis tube is delivered through the transform r 8% to the rectifiercircuit which includes the rectifier tube 82, condenser 83 and resistor84. The rectified current in this circuit passes through the resistor 83 in the direction of the arrow and produces a potential difierenceacross this resistor which is proportional to the strength of the inputsignal substantially throughout the entire audio band.

This potential is applied through the resistor 86 to the input circuitof the amplifier 3?, thus opposing the negative bias which is normallyimposed upon the grids of te tubes 3'ia. In this way the gain ratio ofthe amplifier 3! is made to increase in accordance with an increase ofiiiput signal strength.

The potential drop across the resistor is limited by the action of thediodes tla of the tube 5i, which prevents this potential from increasingbeyond a predetermined value, so that the strongest signals will reducethe bias on thetubes V of the amplifier 3! only to a suitable value whenused for maximum possible output power.

Energy for automatic scratch filter purposes is obtained from thepotentiometer ill and passes through the condenser 98 andauto-transformer $8 to the amplifier tube 92. The energy after beingamplified by the tube 92 is transmitted through the auto-transformer 93and condenser 95 to the rectifier circuit, which includes rectifier tube96 and resistor 97. The condensers 9i! and 95 and the auto-transformers9i and 93 are so chosen as to discriminate against low and intermediateaudio frequencies. The rectified current in this circuit passes throughthe resistor 91 in the direction of the arrow and produces an averagepotential difference across this resistance which is determined by thestrength of the higher frequency portion of the input signal.

When high frequency currents are not present in the input signal thereis a small negative bias on the'grids, of the tubes M and id determinedby the position of the potentiometer '63. these conditions theimpedances of these tubes as and it are low so that the filter networkope crates to prevent the passage of high frequencies, such as scratchand ground noise, to the amplifier 3?, these frequencies beingicy-passed through the condensers 6 3 and ll and the tubes i i and Whenhigh frequencies are present in the input signal however, rectifiedcurrent will fiow through the resistor as already described, thuscausing an increase of negative potential to be produced at the upperend of this resistor as viewed in the figure. This negative potential isapplied through the resistor 98 to the grids of the tubes at and it,thereby increasing their impedance and allowing the high frequencies topass to the amplifier 3?, as under these conditions the resistors 32 and23 will be the controlling factors in preventing the by-passing of thehigh frequencies through the filter network.

Resistors 42 and S3 are several times the impedance of tubes it and 4%and are used to determine the upper possible limit of resistance bridgedacross chokes 5? and 58, and serve to permit serious resonance effectsof condenser 48 and choke and condensers ii and choke 48 when the tubes4 and pass no current.

It is thus seen that as the amount and frequency of the high frequenciesimpressed upon transformer 25 increase, the impedances of the tubes atand it will also increase, thus allowing the filter network to pass thehigh frequencies in accordance with their intensity in the input signal.

No limiting device is necessary for the automatic scratch filter, as anincrease of high frequency signals causes the plate currents of thetubes to be reduced to the minimum possible value of zero.

The potentiometers 29, 3t, 63 and ti l may be operated in unison, sothat with manual increase of signal volume the negative potentialsacross condensers s9 and it will be decreased and increasedrespectively. This will produce less dynamic multiplication and lessautomatic scratch elimination when the circuit is operated at lowervolume level. The voltages across the potentiometers and 6d are derivedfrom the source of piate power 5? through the use of bleed er resistorsE53, iii-i2, Hi3, and Hi, and the potentiometers may be suitably chosento control the plate currents of the tubes between any de- Under Themeters 1 l2 and I I3 are provided to indicate.

the operation of the'circuit as to filtering and dynamic action.

The switches, eliminating either the dynamic multiplier action or theautomatic scratch filter'action. When the switch i9! is closed asuitablebias for normal operation of the push-pull amplifier 31 will beappliedto the grids of the tubes of the amplifier 31 as determined bythe potential of the negative side of the'resistor III.

192 is closed the maximum possible bias from the negative end of thesource of plate power 51 will be applied to the grids of the tubes 44and 46.

The timing of the control circuits is adjusted by the constants of therectifier output systems including condensers 49 and 99 and may besuitable for reasonably fast operation as determined bythe'characteristics of the output circuit. In

general, the operation should be such that the.

control is not noticeable to the earl If it is desired to delay theoperation slightly the switches 193 and l95;are closed, thus throwingthe condensers I94 and I99 in parallel with the condensers 49 and 99. Vi r Condensers 59, 66,'resistors 62, 10, I6 and transformer-8| areadapted to 'make the electrical energy impressed upon rectifier 82correspond to theacoustical value of the output of amplifier 25,

. by discriminating against lower frequency tones.

The filter network comprising condensers B1, 68 and'99; resistors ll,l2, l3 and 14; the autotransformer 9!; the output circuit andautotransformer 93 and condenser are selected to dis-. criminate againstthe lower and intermediate frequency tones. Consequently rectifier 82 isenergized especially by intermediate frequency tones in accordance withthe acoustical volume corresponding to electrical currents in the out-.

put of tube '25, but rectifier 83 is energized by higher frequencytonesin accordance with the currents in the output of tube 25 in the scratchfrequency range. It is evident therefore that energy throughout theentire audio band derived from the sound source is available foroperating the dynamic multiplier control circuits while only the highfrequencies which are in the same range as the background noise of thesound record are used for operating the automatic scratch filtercircuits. The constants of all the circuits including eiernents 99 to 93and condenser 95 maybe so proportioned as to give the desiredfrequency'and energy characteristics for operating the two rectifiersystems for these purposes.

Although one particular type of rectifier system is shown in theaccompanying drawings, it will be understood that any suitable types ofrectifier systems could be used to'produce voltages across condensers 49and 99' suitable for the desired purpose, in accordance with the energyand frequency characteristics.

The entire system therefore, involves an autoinatic scratch filtercircuit controlled by the impedance of vacuum tube devices and a dynamicmultiplier circuit, the gain ratio of which is controlled by thepotentials on the control electrodes. Low and intermediate frequencytones do not effectively operate the scratch filter circuit, but docontrol the dynamic multiplier circult. both the scratch filter and thedynamic multiplier control. This gives a normal amount of expansion ofvolume range for the low and inter- 7 "II and J92 are provided for Whenthe switch High frequency tones, however, operate mediate tones and anincreased expansion for the highest tones. r l g This system provides,through control of potential across condenser '49, dynamic"multiplication or volume expansion on substantially all frequencies,and also provides, through control of potential across condenser99,further dynamic multiplication on a group of selected frequencies inaccordance with their energy content in the input signal. Thetransmission properties of the system maybe proportioned in any mannerdesirable, as for example by choice of transformers and filter elementsin the main amplifying channel, to give substantially uniformtransmission characteristics under conditions of maximum input signal.The use of a low pass filter 28'to' out off signals higher than about5000 cycles, while desirable for disc records, may be omittedwhen filmrecords are used.

Figure 2 depicts the effect of change of poten-,

tial across the condenser 99 upon the change of transmission of signalsthrough the entire amplifier, in an illustrated case. Abscissaerepresent frequency of-signa-ls impressed upon the system, ordinatesrepresent transmission change. The value of transmission with negativepotential across the condenser 99 to produce zero space current ofthetubes 44 and 46 is taken arbitrarily as zero decibels for allfrequencies and is represented by the straightfline.

The curved lineshows the filtering effect when 1norma1 space currentpasses through the tubes 44 and 46, as occurs in the absence of anyappreciable signal, thus indicatingdiscrimination against the highfrequency portions of the signal. 1

When signals are impressed of suitable characteristics to reduce theplatecurrents to Zero, the transmission of the system is altered by thechange of the automatic filter network in such a manner as to increasethe transmission on all frequencies above 360 cycles and .very greatlyon all frequencies above 1000 cycles. The trans mission on frequenciesbetween 60.and 350 cycles is at the same time diminished due to theseriesresonance eifect'of the chokes and condensers in the shunt arms ofthe filter network.

For intermediate values of potentials and space currents thetransmission curve would lie between the two curves depicted. It istherefore seen that any change in the signal which increases the 1negative potential across. .the condenser 99 increases the contrast ofthe high and low frequency'portions of energy present in the signal.

In Figure 3 is depicted the relative effects of signals of differentfrequencies in causing the low frequencies of the same intensity andthat 7 1. In a system for reproducing sound from a sound record, avolume control device for manually controlling the input signalstrength, a variable filter having variable impedance devices adapted tocontrol the frequency transmission characteristics of said system. meansresponsive to a characteristic of the input signal'for controlling 'saidvariable impedance devices, and means associated with said volumecontrol device for altering the control of said variable impedancedevices in proportion to the setting of said volume control device;

2. In a system for reproducing sound from a sound record, a maintransmission channel having a volume control device, a filter adaptednormally to suppress the high frequency components,'and an amplifier, acontrol channel actuated-in accordance with the input signal, saidcontrol channel having means responsive to sub stantially the entireaudio frequency range of said signal for varying the gain ratio'of saidamplifier in accordance with the average intensity of said signal, meansresponsive to the high frequency components of said signal for causingsaid filter to pass said components when they are present in excess of apredetermined strength, and means to selectively render said filter andcontrol circuit inoperative. g

3. In an electrical transmission channel, a variable filter including aspace discharge device connected to control the transmissioncharacteristics thereof and having a control electrode, means responsiveto the signal strength to impress a variable biasing potential on saidcontrol electrode whereby the transmission characteristics of saidfilter vary according to signal strength and means to selectivelyimpress a fixed biasing potential on said control electrode to cause thefilter characteristics to remain constant.

4. In an electrical transmission channel, a variable filter having shuntcondensers and space discharge devices having their space current pathsconnected in series with said condensers to control by their impedancethe transmission characteristics of said filter and having controlelectrodes, means responsive to the signal strength to impress avariable biasing potential on said control electrodes whereby thetransmission characteristics of said filter vary according to signalstrength and means to selectively impress a fixed biasing potential onsaid control electrodes to cause the filter characteristics to remainconstant.-

5. In an electrical transmission channel, a space discharge amplifierhaving control electrodes, a variable filter including space dischargedevices connected to control the transmission characteristics thereofand having control electrodes, means responsive to the signal strengthto impress a variable biasing potential on the control electrodes ofsaid amplifier and said filter so as to vary the amplification factor ofsaid amplifier and the transmission characteristics of said filter inaccordance with signal strength and means to selectively impress a fixedbiasing potential on said control electrodes to cause the amplifier tooperate with a constant amplification factor and to cause the filterremain constant.

6. In an electrical transmission channel, a

space discharge amplifier having control electrodes, a variable filterhaving shunt condensers and space discharge devices having their'spacecurrent paths connected in series with said condensers to control bytheir impedance the transmission characteristics of said filter andhaving control electrodes, means responsive to the signal strength toimpress a variable biasing potential on the control electrodes of saidamplifier and said filter so as to vary the amplification factor of saidamplifier and the transmission characterischaracteristics to 5 ticsofsaid filter in accordance with signal strength and means to selectivelyimpress a fixed biasing potential on said control electrodes to causethe amplifier to operate with a fixed amplification factor and thefilter characteristics to remain constant.

7. In an electrical transmission channel, a space discharge amplifierhaving control electrodes, a variable filter normally adapted todiscriminate against high frequencies, a spacedischarge device connectedto control the discrimination characteristics of said filter and havinga control electrode, means responsive to the signal strength to impressa variable biasing potential on the control electrodes of saidamplifierso as to cause the amplification factor of said amplifier to-vary inaccordance with signal volume and means responsive to the high frequencycomponent of the signal to impress a variable biasing potential on thecontrol electrode of said filter so as to control'the discriminationcharacteristics thereof in accordance with the strength of said highfrequency components and means to selectively impress a fixed biasingpotential on all of said control electrodes to cause the amplifier tooperate with a fixed amplification factor and the filter characteristicsto remain constant.

8. In an electrical transmission channel, a variable filter having shuntcondensers, space discharge devices having their space current pathsconnected in series withsaid condensers to control by their impedancethe transmission characteristics of said filter and a high resistanceconnected in parallel to said space discharge devices to preventresonance effects in said condensers when said space discharge devicesoperate at a high impedance.

9. In an electrical transmission channel, a variable filter comprising acapacity resistance network having series resistances and shuntcapacities, space discharge devices having their space current pathsconnected in series with said capacities to vary by their impedance theeffectiveness thereof, whereby the said network constitutes a low passfilter when the internal impedance of said space discharge devices islow and constitutes an attenuation network without appreciablediscrimination when said impedance is high.

I 10. In an electrical transmission channel, a filter network includingimpedances connected in the shunt leg thereof and a pair of spacedischarge devices connected in push-pull relationship in series withsaid impedances and adapted to control by their internal impedance theeffectiveness of said first impedances.

'11. In an electrical transmission channel, a filter network includingcondensers connected in the shunt leg thereof, a pair of space dischargedevices connected in push-pull in series with said condensers andadapted to control by their impedance the efiectiveness thereof.

12. In an electrical transmission channel, a filter network comprising aseries impedance and a shunt leg on each side of said impedance, each ofsaid shunt legs including a pair of condensers and a pair of spacedischarge devices connected in push-pull relationship with their spacecurrent paths in series with said condensers and means controlling theimpedance of said space discharge devices to Vary thereby theefiectiveness of said condensers.

13. In an electrical transmission channel, a filter network comprising aseries impedance and a shunt leg on each side of said impedance, each ofsaid shunt legs including a pair of condensers and a pair of spacedischarge devices connected in push-pull relationship with their spacecur-r rent paths in series with said condensers, means controlling theimpedance of said space discharge devices tovary thereby theefiectiveness of said condensers and means to apply a high fixed biasingpotential to said space discharge devices'to substantially block thesame whereby said condensers are rendered ineffective.

14. In an electrical transmission channel; a filter network comprising aseries impedance and a shunt leg on each side of said impedance, each15. ,In an electrical transmission'channel, a resistance attenuationnetwork including a'shuiit potentiometer, means a series and a shuntresistance in said network to decrease ducing substantial changecharacteristics of said network.

deriving a control voltage from said potentiometer and means tointroduce;

said control voltage without introin the transmission 16. Ina system forreproducing sound from a sound record, a space discharge amplifierhaving control electrodes, a variable filter including space dischargedevices connected to control the transmission characteristics thereofand having control electrodes, means responsive to the signal strengthto impress -a variable biasing potential ,on the control electrodes ofsaid amplifier and said filter so as to vary the amplification factor ofsaid amplifier and the transmission characteristics of'said filter inaccordance with signal 7 strength; volume control means for adjustingthevolume of the reproduced sound and means actus ated in unison with saidvolume control,'means to adjust the initial biasing potential of saidspace V discharge devices so that variations in signal strength produceless control effect when the volume control is set for a lower volume.

ELLISON S. PURINGTON.

